The present invention relates to an implant fixation stem such as for a prosthetic hip implant. More specifically this invention is concerned with controlling stem stiffness and stem implantation characteristics.
It is advantageous in hip replacement surgery to use an implant that fills the interior space of a femur as much as possible. To this end, a particular hip stem design is usually provided as a system having a variety of sizes. During surgery, the femur is hollowed with a rasp and the appropriately sized implant is then driven into the hollowed femur. However, the femur is curved so that there exists a tradeoff between the stem filling the femur and the stem being able to traverse the curve without undue resistance to implantation and pressure on the femur. It is known to reduce the stiffness of a stem by various means in order to ease implantation and avoid splitting the femur as it traverses the femoral curve. This is only a partial solution though, because as the stem size increases the stem stiffness increases, generally at an exponential rate. Therefore, larger stems are increasingly more difficult to implant. The prior art has failed to appreciate that it is important not only to control individual implant stiffness, but to control the relative stiffness between implants within an implant system having a range of stem sizes. This is so because as a surgeon is driving a stemmed implant into a hollowed bone he relies on the feedback of how easily the implant enters the hollow bone to determine if femoral damage is imminent and if the implant is well seated. With prior devices the feedback from a small implant is different from that of a large implant, making it difficult for the surgeon to develop a familiarity with the implantation procedure.